The present invention relates to a noise filter to be used in a signal transmission circuit, power supply circuit and the like in various electronic equipment for suppressing noise passing therethrough.
In digital electronic equipment such as computers, noise filters are widely used in signal transmission circuits, power supply circuits, etc.
One example of conventional noise filters comprises a pair of spiral electrodes 21,22 disposed in opposed relationship to each other by way of a dielectric member 20. Both terminals 23,24 of one spiral electrode 21 are connected for inserting the spiral electrode 21 in series in a signal transmission circuit while either one of the terminals 25 of the other spiral electrode 22 is grounded as illustrated in FIGS. 7a through 7c An equivalent circuit of the noise filter is illustrated in FIG. 8. Distributed capacitors formed between the pair of spiral electrodes 21,22 and the inductance of -the one spiral electrode 21 forming the signal transmission circuit provide a low pass filter having a sharp cut-off frequency.
In such conventional filters, ceramic, plastics, glass, mica, etc. are used as tile dielectric member 20. Spiral electrodes are formed on the opposite faces of the dielectric member 20 by the printing process.
However, as portable electronic equipment such as portable telephone, portable personal computer, etc. handling digital signals become popular, more compact noise filters are required. Unfortunately, in the conventional noise filters, there is a certain limit in increasing distributed capacitance by reducing the thickness of the dielectric member and forming longer spiral electrodes by the printing technique, thereby making it difficult to achieve significant compact noise filters.
It is therefore an object of the present invention to provide noise filters suited for quantity production, significantly compact design and laving excellent noise suppressing performance.